The present invention relates to a lighting device in which an amount of light from an illumination lamp is detected by a light receiving element and then an input voltage for the illumination lamp is controlled commensurate with a variation in output of the light receiving element.
In order to prevent a variation in amount of light from an illumination lamp due to a variation in the voltage of a potential source, it has been a common practice to use a lighting device which is controlled such that an amount of light emitted by an illumination lamp is detected by a light receiving element and an input voltage for the illumination lamp is controlled in correspondence with the variation in amount of light detected. The control is provided since the variation in the light leads to a variation in the electrical resistance of the light receiving element which in turn is used to vary the input voltage.
A lighting device of the above-noted type as utilized in the prior art is shown in FIG. 1. A light receiving element 1, such as a CdS cell, is arranged to receive light from an illumination lamp L. The light receiving element is connected to a constant voltage source via terminal B, a rectified voltage source via terminal A, resistors R1 to R10, transistors Tr1 to Tr3, programmable unijunction transistor PUT and condenser C1. With this circuitry, the phase control is effected commensurate with an amount of light emitted by illumination lamp L by using an input of the PUT as a trigger pulse for a bi-directional thyrister TRIAC, which controls the phase of an input A.C. potential source D-E for illumination lamp L.
The above phase control is carried out by the following circuit arrangement. A wave form of a voltage from an A.C. potential source which has been full-wave rectified is fed as an input to terminal A (see FIG. 2). The voltage at terminal A in turn is connected by way of resistor R1 to the base of transistor Tr1. The collector of transistor Tr1 is connected by way of resistor R2 to terminal B of the constant voltage potential source and to the base of transistor Tr2 by way of resistor R3, so as to form a switching circuit with transistors Tr1 and Tr2. Light receiving element 1 also forms a bridge circuit with the aid of resistors R4, R5, R6, R9 and R10. The base of transistor Tr3 is connected to a junction between light receiving element 1 and resistor R9. The other terminal of element 1 is coupled to light responsive variable resistor R5 through resistor R4 and then to resistor R8 and finally to the emitter of transistor Tr3. The collector of transistor Tr3 is connected to condensor C1 and to the anode of programmable unijunction transistor PUT. In addition, the collector of transistor Tr2 together with the gate of transistor PUT is connected to the junction between resistor R6 and resistor R7. The cathode of transistor PUT is connected to an input terminal of a primary winding of a pulse transformer TP. An output terminal of a secondary winding of pulse transformer TP is connected by way of diode D2 and resistor R13 to a gate of the TRIAC. Illumination lamp L is connected via the TRIAC to A.C. potential source terminals D-E. Meanwhile the emitters of the transistors Tr1, Tr2 are connected to ground.
The charge stored in condenser C1 is discharged through the cathode of programmable unijunction transistor PUT, when the anode voltage of the transistor increases beyond the sum of the gate voltage and the voltage drop across the gate and the anode. A resulting discharge current generates a pulse voltage on the gate of the TRIAC by way of pulse transformer TP, thereby causing the TRIAC to be conducting.
During operation of the above-described circuit arrangement, an A.C. wave form as shown at (a) in FIG. 2(a) is fed to terminals D-E, a rectified wave form shown at (c) in FIG. 2(b) is fed to terminal A, and a constant voltage shown at (d) is fed to terminal B. The rectified wave form (c) has a zero phase at zero-phase points (T0, T1, T2 . . . ) of the A.C. wave form (a). A description will be given below of the phase control for one cycle of an A.C. wave form (a).
When illumination lamp L is lit, a lighting switch SW1 is closed by suitable means. When voltages having wave forms (a), (c) are impressed on the terminals D-E and A, respectively, then the base voltage at transistor Tr1 increases simultaneously with the voltages (a) and (c), thereby causing transistor Tr1 to become conducting. Activation of transistor Tr1 nullifies the base voltage at transistor Tr2, thereby causing transistor Tr2 to be turned off, so that a divided-voltage wave form (f) of a constant-voltage wave form (d) is impressed on the gate of transistor PUT. On the other hand, a voltage generated across a voltage divider formed by the resistance of light receiving element 1 and resistors R8, R9 is impressed on the base of transistor Tr3, while a voltage generated across a voltage divider formed by resistor R4, variable resistor R5, resistors R6, R7 is impressed on the emitter of transistor Tr3, so that transistor Tr3 is brought into a conducting condition due to the difference in divided voltages. As a result, condenser C1 is charged by way of transistor Tr3. The wave form of the voltage across the opposite ends of condenser C1 is shown at (e). When the voltage across condenser C1 rises to a linear proportion of the gate voltage of transistor PUT, then the charge stored in the condenser is instantaneously discharged from the cathode of transistor PUT. The discharge of the condenser causes a current which generates a pulse voltage provided to pulse transformer TP. A pulse voltage is then impressed on the gate of the TRIAC thereby triggering the TRIAC (at point (t0 of FIG. 2). Once the TRIAC is triggered, an electric power is supplied to illumination lamp L (during a period shown by the hatched portion (b) in FIG. 2). Thereafter, the TRIAC maintains its conducting condition until an A.C. wave form (a) is brought to a zero phase again (point T1), due to the nature of the TRIAC. During that time, condenser C1 repeats a cycle of charge and discharge, although the charging rate depends on the resistance of light receiving element 1 and the set resistance of variable resistor R5. Consequently, the discharge cycle of condenser C1 is not always coincident with the zero phase of the A.C. voltage wave form (a). The rectified voltage wave form (c) however is also brought to a zero phase, at the time of the zero phase of A.C. voltage wave form (a), so that the base voltage at transistor Tr1 is nullified so as to turn the transistor off. Upon turning off transistor Tr1, a voltage at resistor R2 is instantaneously impressed on the base of transistor Tr2 so as to cause the transistor to start conducting. Meanwhile, the gate voltage at programmable unijunction transistor PUT instantaneously drops to zero, as shown at (f) in FIG. 2. As a result, irrespective of the voltage across condenser C1, the stored charge in the condenser is discharged and the wave form (e) of the voltage across the condenser is necessarily nullified at the zero phase point of the A.C. voltage wave form (a), as shown, thereby bringing the trigger pulse for a phase control into synchronism with the input voltage wave form (a). When the A.C. voltage wave form (a) is built up from a point T1 in the direction of an inverse voltage, the TRIAC is triggered at a point t1 in the manner as previously described. The TRIAC, therefore, maintains its conducting condition up to a point T2, thus repeating these operations for each cycle of A.C. potential source.
The phase control of illumination lamp L may be carried out by utilizing a variation of the electrical resistance of light receiving element 1, which variation is caused due to a variation in amount of light emitted by illumination lamp L. For instance, if the input voltage at the A.C. potential source terminals D-E is lowered for one reason or another, thus causing a corresponding decrease in the amount of light being emitted, then the resistance of light receiving element 1 will be increased. With this increase in resistance, the charging rate for condenser C1 will be increased, while the timing pulses to trigger the TRIAC will be more rapidly supplied. As a result, the electric power to be impressed on illumination lamp L will be increased, thereby compensating for any instantaneous drop in amount of light.
The desired level of light to be emitted by lamp L can be set by varying the resistance of variable resistor R5, which is arbitrarily preset by the user.
With a lighting device using the described light receiving element, when a potential source voltage is impressed on illumination lamp L, then the lamp will be lit in accordance with the described phase control. During the stage of lighting in which the lamp is building up to the desired level of illuminescence, i.e., during the time required for the lamp to reach such a level, very little light is incident on light receiving element 1.
Consequently, during this period, the electrical resistance of light receiving element 1 is extremely high and hence the rate of the timing pulses to trigger the TRIAC is highly accelerated, so that electrical power approximating equal to the full power of the source may be supplied to the illumination lamp.
Furthermore with this arrangement, the resistance which is provided may not be accurately commensurate with the variation in amount of light from lamp L, during the build up stage of lighting of the lamp. This failure occurs since light receiving element 1 requires a given duration of time for building up to its responsiveness to a variation in the amount of light, when the amount of light was initially of a zero level, or darkness.
In FIG. 3, the variation in voltage to be impressed on illumination lamp L is shown, with the voltage to be impressed being represented along the ordinate and the time being represented along the abscissa. As shown, during the build up stage (.DELTA.t) of lighting, an excessive voltage or overvoltage is supplied to the input of the lamp, so that the amount of light being emitted, or the brightness of the lamp, rapidly increases. As shown in FIG. 3(b), in which the amount of light is represented along the ordinate and the time is represented along the abscissa, the amount of light goes beyond a rated amount of light, i.e., causes an overshooting phenomenon, after which the amount of light enters a steady, controlled zone. The overshooting can also cause the amount of light to oscillate as shown by a broken line in FIG. 3(b) before leveling off to a steady state.
This overshooting phenomenon which occurs due to an overvoltage input shortens the service life of the illumination lamp and also can cause over-illumination during the build up stage of lighting. This is particularly troublesome in the case where the exposure lamp in an electrophotographic copying machine is the above-described type of controlled lighting device. Where the exposure lamp in a copying machine repeats a cycle of being turned on and off in association with a copying operation, the overvoltage input at the initiation of lighting will adversely affect the service life of the lamp.